Knitting needles have been around for hundreds of years and different ways of measuring needles sizes have come about. This can make it tricky if you're used to a certain way of measuring or using an old pattern using a certain set of sizes.
All of the needles we sell now use metric sizing. The great news is 4mm is 4mm everywhere - no need to worry about a US 4mm, UK 4mm or Japanese 4mm!
But you do need to make sure you convert correctly between the difference schemes, and you can see our conversion table here. These are approximate, so won't necessarily be exactly the same size - so always do a tension square!
The standard way to measure the size of a needle is by using a gauge. This is basically a bunch of different size holes through which you fit your needle. The smallest hole it fits through is its size.
But there is an issue with gauges: a needle which is very slightly bigger than the hole will measure the next size up.
Surely 4mm is 4mm? Yes, but all manufacturers, of everything, work to tolerances. This is the difference from the target size that is considered by them to be acceptable.
With some manufacturers, for 4mm they may accept 3.8mm to 4mm. Some may accept 3.9mm to 4.1mm. They then make their gauges according to their tolerances. Remember, even the holes in the gauges have tolerances!
What this means in practice is that some brand gauges may measure some tips as different sizes to what their own brand gauges measure. Due to the way gauges work, they'll always be very lenient for too small needles, but very strict on larger needles. So you may find your 4mm needle measures 4.5mm in your gauge, despite measuring 4mm in another gauge.
The solution: always use the same brand gauge as the brand of needles you're using. In the unlikely event the gauge measures the tips as the wrong size, then it's definitely a fault with the tip (or gauge).
It's not just the size of the needle which affects the tension. The material of the needle, the yarn, and type of stitch - even what you are watching on TV - can change it. You should always do a tension square to check your tension before knitting a project. You should use the needles and yarn you'll use on your project. We don't advise changing needles mid-project because it could throw your tension off.
Mostly though these tiny variances in sizes will make no difference to your project as long you stick with them for the project, and do a tension square before you start!